The present invention relates to a turbo decoder for receiving turbo-coded data for error correction in a communication system, correcting transmission data error and decoding the data.
In a communication system, there are adopted various error correction encoding systems for remedy of transmission data error. For example, as described in K. Yamaguchi et al., “Novel coding algorithm closing to Shannon's limit (Turbo code)”, Nikkei Electronics No. 721, pp. 163–177 issued on Jul. 13, 1998 in Japan, there is known a turbo encoding system as an encoding system having high error correction capability.
According to a communication system adopting a turbo encoding system shown in FIG. 18, there are prepared two recursive and systematic convolutional encoders in a turbo encoder 201 and at a first convolutional encoder 204, signals of an information source are encoded in the order of input. Before inputting signals to a second convolutional encoder 206, the signals of the information source are temporarily stored in a memory and by an interleaver 205 for taking out the signals in an order in accordance with a certain pattern, the order of data bits are arranged and the signals are encoded by the convolutional encoder 206. Turbo encoded data UT, YT1 and YT2 are multiplexed into one channel and transmitted from the two encoded outputs to a communication channel 202. Encoded data UR, YR1 and YR2 having passed through the communication channel 202 are inputted to a turbo decoder 203, where decoded data UO is obtained by processing a turbo decoding.
The turbo decoder 203 includes two decoders 207 and 209, two interleavers 208 and 211 and a de-interleaver 210. When received data UR, YR1 are put into the decoder, the decoder carries out soft output decoding. Further, received data YR2 corresponds to the transmitted data YT2 constituted by interleaving and convolutionally encoding an original signal X, and the interleaver 208 interleaves decoded data of the decoder 207 to correspond to the received data YR2, and interleaved data are put into the decoder 209 and soft output decoding is executed. The de-interleaver 210 deinterleaves an output of a decoded result to be in the order of original data to thereby provide the decoded output UO. The decoded output UO is put again into the decoder 207 as received data UR and the operation mentioned above is repeated. By carrying out the decoding processing repeatedly, randomly generated error or error generated in burst can be corrected. As a decoding system of the decoders 207 and 209, a MAP (maximum a posteriori probability) decoding system or a SOVA (soft-output Viterbi algorithm) decoding system is well known. Further, the decoders 207 and 209 are constructed by the same constitution and therefore, actually, a single decoder is frequently used to alternately switch for the decoder 207 (for example, for odd number times) and for the decoder 209 (for example, for even number times).
According to the above-described turbo decoder, by increasing an iteration number of the decoding processing, the correction capability is promoted and errors can be reduced even in a communication channel having poorer quality. However, when the iteration number is increased, an operational processing amount is increased, a higher clock frequency is needed and power consumption is increased.
Particularly, in a mobile station, it is important to restrain power consumption as small as possible to thereby ensure to drive the mobile station for a long period of time by a battery. According to IMT-2000 (International Mobile Telecommunication systems) constituting international standards of a mobile communication system, application of the turbo code is prescribed and in a mobile station in conformity with IMT-2000, there is required a turbo decoder having sufficient error correction capability and small power consumption.
It is an object of the invention to provide a turbo decoder reducing power consumption by reducing a processing amount while ensuring sufficient error correction capability.
In order to achieve the above-described object, a turbo decoder according to the invention is characterized in comprising means for judging a reliability of a decoded result based on statistics of a result of soft output decoding in a decoder and controlling means for controlling the decoder to execute a decoding operation successively repeatedly when a result of judging the reliability of the means for judging the reliability does not satisfy a desired reliability and stop the decoding operation and output a decoded result when the desired reliability is achieved.
The decoding operation which has conventionally been executed continuously by a predetermined number of times regardless of presence or absence of the reliability, is stopped at a middle stage achieving the reliability and accordingly, the processing amount is reduced and the power consumption is reduced in accordance therewith.
These and other objects and many of the attendant advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.